Rutherford’s Gold Foil Experiment Why the Plum Pudding Model Failed Foundations & History

Описание: In this Foundations & History micro‑lecture, we look at Rutherford’s gold foil experiment and understand why it destroyed Thomson’s plum pudding model and led to the nuclear model of the atom.

After J.J. Thomson discovered the electron, he proposed that the atom was a roughly uniform sphere of positive charge with electrons embedded in it like “plums in a pudding”. In such a diffuse positive charge distribution, heavy, positively charged alpha particles should pass through with only very small deflections. No strong backscattering was expected.

Rutherford, with Geiger and Marsden, tested this by firing alpha particles at a very thin gold foil and observing where they hit a fluorescent screen. They found that:

Most alpha particles passed straight through with almost no deflection → the atom is mostly empty space.
A small fraction were deflected by small to moderate angles.
A very tiny fraction (about 1 in 8000) were deflected by large angles, some almost straight back – an “incredible” result in Rutherford’s own words.
The only way to explain such large‑angle scattering was to assume that almost all the positive charge and mass of the atom is concentrated in a tiny, dense nucleus, with electrons orbiting around it. This is the Rutherford nuclear model of the atom.

However, Rutherford’s model still had problems: classically, orbiting electrons (accelerating charges) should radiate continuously and spiral into the nucleus, making the atom unstable, and the model could not explain atomic line spectra. This set the stage for Bohr’s model, which kept Rutherford’s nucleus but introduced quantised orbits and discrete energy levels.

For CBSE & JEE students, this video focuses on:

The basic setup and observations of the gold foil experiment,
The key conclusions: mostly empty space, small dense nucleus, failure of the plum pudding model,
The limitations of Rutherford’s model and how it naturally leads to Bohr’s quantised orbits.
It fits into our “Foundations & History | Conceptual Physics” playlist along with our micro‑lectures on cathode rays, Millikan’s oil drop experiment, and line spectra → Bohr orbits.